The mechanisms behind the alteration of chemical carcinogenesis by the dietary lipotropes, choline and vitamin B12 have been studied. The activation of the carcinogen 2-acetylaminofluorene was increased in rats fed a low lipotrope diet, which also increased its carcinogenic activity. The chronic administration of diethylnitrosamine (DEN) to rats led to an increased uptake of labelled precursors into hepatic lecithin; the uptake of labelled precursors into phosphatidyl ethanolamine and phosphatidyl serine was unaffected by chronic DEN administration. Transformed liver epithelial cells were found to have a higher requirement for methionine than normal liver epithelial cells. Mice bearing leukemia L-1210 survived slightly longer on methionine-deficient diets than on methionine-adeuqate diets. In rapidly proliferating liver methionine reversed the folate-deficiency apparently caused by a high ratio of thymidylate synthetase to dihydrofolate reductase. BIBLIOGRAPHIC REFERENCES: Poirier, L. A. and Boutwell, R. K.: Current problems in nutrition and cancer. An introduction. Federation Proc. 35: 1307-1308, 1976. Buehring, Y.S.S., Poirier, L. A. and Stokstad, E.L.R.: Folate deficiency in the livers of diethylnitrosamine-treated rats. Cancer Res. 36: 2775-2779, 1976.